1. Field of the Invention
This invention relates to an apparatus and method for generation of a control signal for the optimization of sulfur removal in a Claus process unit. More particularly, this invention relates to: (i) an apparatus for generation of a control signal for the optimization of sulfur removal in a Claus process unit, the apparatus comprising means for inducing the flow of a portion of Claus unit tail gas into the apparatus, means for heating the portion of tail gas, means for catalytically oxidizing H.sub.2 S contained within the tail gas to SO.sub.2, and means for measuring the temperature rise associated with the oxidation reaction and converting the measurement to an appropriate control signal to control the rate of air flow into the Claus unit; and (ii) a method of generation of a control signal for the optimization of sulfur removal in a Claus process unit, the method comprising inducing the flow of a portion of Claus unit tail gas into the abovedescribed apparatus, heating the portion of tail gas, catalytically oxidizing the portion of tail gas in the presence of an oxygen-containing gas to cause the oxidation of H.sub.2 S to SO.sub.2, measuring the temperature rise associated with the oxidation reaction, converting the measurement to an appropriate signal, and using the signal to control the rate of air flow into the Claus unit.
2. Information Disclosure Statement
It is well known to those skilled in the art that elemental sulfur may be recovered from H.sub.2 S-containing gas streams by employing the Claus process or modified Claus process. These processes are discussed, for example, in Vol. 22 of the Kirk-Othmer Encyclopedia of Chemical Technology at pp. 276-82 (3d ed. 1983) and H. Paskall, "Basis of the Claus Process" (Western Research--Calgary, Alberta, Canada (Sep. 1981)). Most modern sulfur-recovery plants are based upon the modified Claus process. The modified Claus process typically converts H.sub.2 S to elemental sulfur according to the series of reactions EQU 3H.sub.2 S+.sup.3.sub.2 O.sub.2 .fwdarw.SO.sub.2 +2H.sub.2 S+H.sub.2 O.fwdarw.3S+3H.sub.2 O
where approximately 1/3 of the H.sub.2 S is initially oxidized to SO.sub.2 by combustion with air, and residual H.sub.2 S subsequently reacts with the SO.sub.2 to form elemental sulfur. Approximately 50% of the product sulfur is formed immediately in the Claus process unit combustion zone, with remaining H.sub.2 S conversion accomplished in a series (typically 1-4) of downstream catalytic stages. Tail gas containing residual amounts of H.sub.2 S and SO.sub.2 exits the Claus process unit, and must be further treated prior to discharge into the atmosphere.
Maximum sulfur recovery is achieved when the proportions of air/O.sub.2 and H.sub.2 S conform to the abovedescribed stoichiometry. To achieve the correct stoichiometric ratio, the traditional control approach has been to adjust the air flow rate entering the Claus unit so as to maintain a tail gas H.sub.2 S/SO.sub.2 ratio=2. However, the more recent trend has been to express the tail gas H.sub.2 S/SO.sub.2 concentration control parameter in terms of "excess H.sub.2 S" or "air demand" (H.sub.2 S --2SO.sub.2) or "excess SO.sub.2 " (2SO.sub.2 --H.sub.2 S) The "excess H.sub.2 S" or "excess SO.sub.2 " approach is often preferred over the H.sub.2 S/SO.sub.2 ratio as a control parameter because it yields a control signal linearly proportional to the required change in air flow rate required to achieve correct stoichiometry. References describing methods for Claus unit process control include M. C. J. Beamish, "Controlling the Claus Process" (Western Research-Calgary, Alberta, Canada (Sep. 1982)) and G. W. Taggart, "Optimize Claus Control", Hydrocarbon Processing (Dec. 1980).
From the above, it is clear that accuracy and reliability of analysis of the H.sub.2 S concentration in the tail gas is of fundamental importance in providing for reliable and accurate control of the air flow entering the Claus unit and subsequent optimization of sulfur recovery, as discussed in P. Grancher, "Advances in Claus Technology", Hydrocarbon Processing (Sep. 1978) pp. 257-62. For example, the most sophisticated Claus units use feed forward control schemes, whereby continuous onstream analysis of the feed gas (in addition to tail gas analysis) serves as a basis for automatic incoming air flow rate adjustment to compensate for changes in gas quality. However, feed forward control is sensitive to errors in input data, and therefore requires accurate gas analysis, as described in G. Taggart, "Be Careful of Feedforward for Claus Control", Hydrocarbon Processing (Mar. 1981). Thus, there is a need for an accurate, reliable and cost-effective means of generating a control signal for control of air flow into the Claus unit by analyzing tail gas composition.
Various methods of analyzing Claus unit tail-gas composition are discussed, for example, in J. W. Palm & N. M. Caruthers, "Guidelines And Control of SRU Tail Gas Composition", Oil and Gas Journal (Nov. 20, 1978), pp. 151-55. These methods include older titration techniques such as the Tutweiler method and on-line analysis techniques such as gas chromatography and ultraviolet spectrophotometry. On-line gas composition analyzers, including flammable-vapor analyzers, are discussed for example in J. T. Y. Yeh, "Online Composition Analyzers", Chemical Engineering (Jan. 20, 1986), pp. 55-68. Flammable-vapor analyzers feed a sample of flammable vapor, along with air, into a controlled flame. The vapor then burns, releasing heat measured by a temperature detector. The concentration of flammable vapor is determined by the measured temperature. In another version, air containing flammable vapors is passed over a temperature detector coated or encased in a catalyst which causes combustion and a certain temperature increase that correlates with flammable-vapor concentration. On-line ultraviolet spectrophotometers for use in continuous monitoring of sulfur recovery units are described, for example, in the product literature of Du Pont Model 4620 Sulfur Recovery Unit Analyzer System (available from Du Pont Company--Analytical Instruments Division) and Western Research Model 700 Air Demand Analyzer System (available from Western Research, a division of Bow Valley Resource Services Ltd. (Canada)).
The method and apparatus of the instant invention relate to the generation of a control signal for the optimization of sulfur removal in a Claus process unit. The signal is generated by oxidizing a portion of the Claus unit tail gas stream in the presence of a catalyst capable of oxidizing H.sub.2 S contained within the tail gas stream to SO.sub.2, measuring the temperature rise associated with the oxidation reaction, converting the measurement to an appropriate control signal, and using the signal to control air flow rate into the Claus unit to achieve stoichiometry. The catalytic oxidation of sulfur to SO.sub.2 is described, for example, in:
G. J. Shugar, R. A. Shugar, and L. Bauman, Chemical Technicians' Ready Reference Handbook, p. 258-60, which discloses a method of determining sulfur content by conversion of organic sulfur to SO.sub.2 via high temperature (1300.degree. C.) combustion in the presence of pure oxygen and a V.sub.2 O.sub.5 catalyst;
U.S. Pat. No. 4,279,882 (Beavon), which discloses a process for sulfur production which is applicable to the treatment of H.sub.2 S -containing gas streams, the process comprising contacting at a temperature below 850.degree. F., an H.sub.2 S -containing gas stream with oxygen or air and a recycle gas containing H.sub.2 S and SO.sub.2 in the presence of a selective oxidation catalyst capable of selectively oxidizing H.sub.2 S to SO.sub.2 without formation of SO.sub.3, the catalyst preferably comprising a vanadium oxide (e.g. V.sub.2 O.sub.5) on a non-alkaline porous refractory oxide; and
U.S. Pat. No. 4,171,347 (Hass), which discloses a catalytic for conversion of H.sub.2 S to SO.sub.2, the catalyst comprising one or more vanadium oxides or sulfides (e.g. V.sub.2 O.sub.5) on a non-alkaline porous refractory oxide. The catalyst selectively oxidizes H.sub.2 S to SO.sub.2 in the presence of air, H.sub.2, CO, light hydrocarbons, and ammonia. The catalytic reaction is highly exothermic, with the reactor temperature rising proportionately to the concentration of H.sub.2 S in the feed gas-oxidant mixture; and
R. H. Hass et al., "Process Meets Sulfur Recovery Needs," Hydrocarbon Processing (May 1981), pp. 104-07, which discloses a catalytic process for gas stream sulfur recovery.